The present invention relates to a flexible tubular conduit of the type having at least one tubular layer constituted by at least one shaped band of a cross section substantially in the form of an elongated S, helically wound with a small pitch and with its successive spirals interlocked.
The applicant company has for many years manufactured and sold flexible tubular conduits of great length, of high mechanical strength, useful particularly for carrying hydrocarbons, particularly in connection with undersea petroleum producing installations.
Such flexible conduits generally comprise a tubular layer constituted by a double-interlocking strip, this layer being able to constitute the internal body of the flexible conduit in "rough bore" type conduits, an armor designed to give the flexible tubular conduit great resistance to internal pressure and to traction, an intermediate sealing sheath and, if necessary, an external protective sheath.
In conduits of the "smooth bore" type the intermediate sealing sheath is replaced by an internal sheath or tube of plastic or elastomeric material.
To manufacture such tubular layers, a flat metal strip, such as stainless steel strapping, is deformed plastically to make a shaped or profiled strip having a crimpable cross-sectional shape, and this strip is wound helically with a small pitch and with interlocking of the successive spirals. The flexibility of the tubular layer is obtained by the existence of a certain axial free play between the successive crimpable spirals. The shaped or profiled strip has a cross section formed of a first and a second principal coaxial cylindrical span spaced radially apart and joined together by a central span that is inclined with respect to the longitudinal axis of the conduit, each of the first and second principal spans being joined by a first and second convex radial span, respectively, to a first and second secondary span, respectively, at least one of the said secondary spans being cylindrical, coaxial with respect to the principal spans, and spaced radially away from the latter.
Also known are conduits of a first type of construction comprising a tubular layer constituted by a molded strip whose both secondary portions are cylindrical, for example the flexible crimped metal tubes defined by Standard NF E-29-828.
A more or less great radial free play can exist between the different confronting spans such that the resistance to crushing radially is relatively low.
In a second type of construction, the first secondary portion is cylindrical, while the second secondary portion is constituted by a concave axial section whose concavity faces in the opposite direction from the second principal portion and which constitutes a prolongation of the end of the convex radial portion which prolongs the said second principal portion.
An illustration of this type of tubular layer is given in French patents A-2,569,462, A-2,559,920, or A-1367139.
In a variant of this type of construction, provision is made for inserting a gasket, particularly of copper or asbestos, in the fold formed between the axial, concave end section and the first principal span of the adjacent spiral. An illustration of this type of tubular layer is given in French standards NF-E-29-829, /-832 or /-833.
In the type of construction comprising a concave axial section, the contact between the second secondary span of one spiral and the first principal span of the adjacent spiral must normally be made only at the zone of transition between the convex radial portion and the concave axial end section.
Consequently it has been found that the resistance to crushing of the shaped tubular layer and of the flexible tubular conduit incorporating this layer may prove insufficient when the mechanical stresses which it must withstand are great, particularly in applications where a high pressure is applied to the exterior of the conduit. Thus, mention can be made of the case of flexible tubular conduits used for carrying hydrocarbons in deep waters where a high hydrostatic pressure prevails, where the tubular layer has to withstand the crushing effect due to the hydrostatic pressure and to the induced effects of the loads supported by the traction fittings which are stressed by the considerable weight of the conduit.
One condition necessary for obtaining great resistance to crushing, as well as good performance in the case of dynamic effects, is that, on the one hand, the tubular layer be made with a precise interlocking, without radial free play between the confronting portions of the adjacent spirals, and that, on the other hand, the cross section of the shaped strip have an actual configuration in conformity with its theoretical definition, without having defects such as excessive zones of curvature. In practice, to prevent the risk of radial free play, the tubular layers are made with a limited radial clamping effect. As a result of the very close tolerances thus imposed on the finished product, the industrial production has to be performed with a special care which increases costs.